The present invention relates to a spark plug having a novel center discharge electrode and a manufacturing process thereof.
In the prior art, conventional types of spark plugs have a metallic center discharge electrode (center electrode hereinafter) which is formed of a nickel rod with a copper core or a nickel rod to which a tip of Au-Pd alloy is welded. In the former type, a high dimensional accuracy for each parts, particularly that for an inner diameter of a discharge tip portion of a refractory insulator (insulator hereinafter) and for an outer diameter of the center electrode is required. Furthermore, a sealing process for these parts should be controlled in a highest accuracy in respect to sealing and impact resistance properties of such products. Those are marked as drawbacks of this type of the spark plug.
In the latter type, an improved durable life is obtained, whereas it requires an additional step of welding and a larger amount of discharge tip metal is consumed rather than actually necessary for discharge per se although merely a minor amount thereof functions as such in the practice. The discharge tip is usually welded to the head portion with an enlarged diameter of a base electrode rod formed in a rivet like (T type cross-section) form, which is necessary for preventing the discharge electrode from dropping off in order to mechanically form-lockingly secure it to a discharge end bore of the insulator, because a detriment to bonding strength of welded portion during an application under a high temperature would otherwise be caused. Those facts make this type also accompanied with a higher cost of manufacture.